The invention relates to a chemical sensor tool for use in downhole analyzing of fluids produced from subterranean formations. More specifically it relates to a potentiometric sensor for downhole pH and ion content analysis of effluents produced from subterranean formation
Analyzing samples representative of downhole fluids is an important aspect of determining the quality and economic value of a hydrocarbon formation.
Present day operations obtain an analysis of downhole fluids usually through wireline logging using a formation tester such as the MDT(trademark) tool of Schlumberger Oilfield Services. However, more recently, it was suggested to analyze downhole fluids either through sensors permanently or quasi-permanently installed in a wellbore or as through sensor mounted on the drillstring. The latter method, if successful implemented, has the advantage of obtaining data while drilling, whereas the former installation could be part of a control system for wellbores and hydrocarbon production, therefrom.
To obtain an albeit crude estimate of the composition of downhole fluids, the MDT tools uses an optical probe to estimate the amount of hydrocarbons in the samples collected from the formation. Other sensors use resistivity measurements to discern various components of the formations fluids.
Particularly, knowledge of downhole formation (produced) water chemistry is needed to save costs and increase production at all stages of oil and gas exploration and production. The following applications are of interest:
Prediction and assessment of mineral scale and corrosion;
Strategy for downhole oil/water separation and water reinjection;
Understanding of reservoir compartmentalization/flow units;
Characterization of water break-through;
Derivation of Rw.
Some chemical species dissolved in water (like, for example, Clxe2x88x92 and Na+) do not change their concentration when removed to the surface either as a part of a flow through a well, or as a sample taken downhole. Consequently information about their quantities may be obtained from downhole samples and in some cases surface samples of a flow. However, some chemical species, such as H+(pH=-log[concentration of H+]) CO2, H2S and water parameters, such as ORP (redox potential or Eh), do change significantly while trip to the surface. This change occurs mainly due to a huge difference in temperature and pressure between downhole and surface environment. In case of sampling, this change may also happen due to degassing of a sample (seal failure), mineral precipitation in a sampling bottle, and (especially in case of H2S)xe2x80x94a chemical reaction with the sampling chamber. It should be stressed that pH, H2S, CO2, and ORP are among the most critical parameters for corrosion and scale assessment. Consequently it is of considerable importance to have their downhole values precisely known.
Hence, there is and will continue to be a demand for downhole chemical measurements. However, no downhole chemical measurements actually performed in an oil and gas producing well have been reported so far.
To meet demand for chemical measurements of increasing accuracy, it may appear obvious to adapt chemical analysis tools known from chemical laboratory practice to the hostile environment of a subterranean borehole. Such known analysis tools include for example the various types of chromatography, electrochemical and spectral analysis. Particularly, the potentiometric method has been widely used for the measurements of water composition (pH, Eh, H2S, CO2, Na+, Clxe2x88x92 etc. . . . ) both in the laboratory and in the field of ground water quality control. However, so far the environmental conditions within a subterranean wellbore rendered attempts to perform such measurements under real hydrocarbon wellbore condition purely theoretical.
Among the known state of the art in the field of high temperature potentiometric are: Diakonov I. I., Pokrovski G. S., Schott J., Castet S., and Gout R. J. -C. xe2x80x9cAn experimental and computational study of sodiumxe2x80x94aluminum complexing in crustal fluidsxe2x80x9d, in: Geochim. Cosmochim. Acta, 60(1996), 197-211 and Midgley D. xe2x80x9cA review of pH measurement at high temperaturesxe2x80x9d, in: Talanta, 37(1990), 8, 767-781.
General downhole measurement tools for oilfield applications are known as such. Examples of such tools are found in the U.S. Pat. Nos. 6,023,340; 5,517,024; and 5,351,532 or in the International Patent Application WO 99/00575. An example of a probe for potentiometric measurements of ground water reservoirs is published as: Solodov, I. N., Velichkin, V. I., Zotov, A. V. et al. xe2x80x9cDistribution and Geochemistry of Contaminated Subsurface Waters in Fissured Volcanogenic Bed Rocks of the Lake Karachai Area, Chelyabinsk, Southern Uralsxe2x80x9d in: Lawrence Berkeley Laboratory Report 36780/UC-603(1994b), RAC-6, Ca, USA.
It is therefore an object of the present invention to provide apparatus and methods to perform potentiometric measurements in a subterranean wellbore for hydrocarbon exploration and production.
The invention achieves its objects by providing a potentiometric sensor with at least one reference and one measuring electrode having a permanent aqueous contact between measuring and reference electrode. The contact is preferably ensured by discharging the internal solution from the reference electrode directly onto a measuring (ion-sensitive) membrane and protecting the reference junction of the reference electrode with water wet porous material, such as sintered glass.
In a preferred embodiment, the stability of a signal/electrode fouling can be checked by measuring a signal with and without an additional resistance.
A potentiometric technique can be applied for example as part of a production logging tool and open hole formation tester tool (Modular Dynamic Tester, MDT). In the latter case, the technique can provide a downhole real-time water sample validation or downhole pH and H2S measurements for prediction of mineral scale and corrosion assessment. A new gas sensing combination potentiometric sensor is also proposed for simultaneous detection of H2S and CO2 partial pressures in any fluid (oil, gas, water).
These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the following detailed description and drawings.